C FE: use correct location range for static assertions

C FE: use correct location range for static assertions

gcc/c/ChangeLog:
	* c-parser.c (c_parser_static_assert_declaration_no_semi): Use the
	expression location, falling back on the first token location,
	rather than always using the latter.

gcc/testsuite/ChangeLog:
	* gcc.dg/diagnostic-range-static-assert.c: New test case.

From-SVN: r231704